The present invention relates to an oil pump for an automatic transmission, and more particularly to an effective measure for removing residual pressure occurring in the interface defined between a pump housing and a pump cover.
In an automatic transmission, an oil pump is an indispensable element as a source of pressurized oil supplied to a control valve assembly for controlling gear shifting. Commonly, an oil pump comprises a pump housing formed with a circular recess receiving therein pump elements and a pump cover secured to the pump housing to close the recess. The interface defined between the pump housing and the pump cover is sealed. Under an operating condition when a discharge pressure is high, a portion of oil discharged out of a pump discharge port will enter the interface between a pump housing and a pump cover, causing a residual pressure to occur. This residual pressure tends to separate the pump housing from the pump casing to increase the clearance therebetween, causing a substantial amount of oil leakage which results in a considerable drop in discharge performance of an oil pump.
In the case of an oil pump employed by a known transaxle of the A240E type manufactured by Toyota Motor Company Limited, a plurality of drain grooves are open at the interface between a pump housing and a pump casing for removing oil having entered the interface. These drain grooves provided for this purpose only are formed in the interface, so that the interface has to have a relatively large area for allowing arrangement of the drain grooves in addition to passages and/or grooves which are needed for gear shifting and torque converter operations.
An object of the present invention is to remove residual pressure occurring between a pump housing and a pump cover without relying on the above-mentioned measure of forming additional drain grooves used only for the drainage purpose.